1. Field of the Invention
The present invention relates to cementing plugs for use in the cementing of casing in oil and gas wells. More specifically, the present invention relates to cementing plugs that are particularly well suited for use with a polycrystalline diamond compact (PDC) drill bit. The cementing plugs of the present invention may be configured as bottom cementing plugs or top cementing plugs.
2. Description of the Prior Art
It is well known in the art to conduct oil and gas well cementing procedures with a cementing plug container assembly that is designed to contain one or more cementing plugs for injection into the casing of a well. Typically, a first cementing plug is injected prior to the introduction of cement into the casing and a second cementing plug is injected to displace the cement through the casing and to cement the casing in the well. Conventional cementing plugs are made of an aluminum or plastic core and a finned outer shell made of rubber.
Typically, drilling operations are resumed after the casing is cemented in the well. When drilling operations are resumed, the drill bit must first pass through the cementing plugs which are lodged at the lower end of the cemented casing. Conventionally, the cementing plugs and any residual cement are drilled out and removed from the casing with tooth-type rock bits. The teeth on a conventional rock bit are effective in drilling through the conventional cementing plugs made of aluminum and rubber even though the plugs are free to rotate within the casing.
A polycrystalline diamond compact (PDC) drill bit has been introduced to the drilling bit art that advantageously replaces tooth-type rock bits under certain conditions. PDC drill bits, however, do not drill through conventional cementing plugs made of aluminum and rubber as effectively as conventional tooth-type rock bits. Instead, PDC drill bits tend to spin the cementing plugs within the casing. To overcome this problem it has been proposed to use a non-rotating plug set as disclosed in U.S Pat. Nos. 4,836,279 and 4,858,687. These attempts, however, to overcome the incompatibility, from a drilling standpoint, of PDC drill bits and cementing plugs have been largely unsuccessful.